The present invention relates in general to substrates for facilitating epitaxial growth thereon of defect-free, lattice mismatched materials, and a process for fabrication of these substrates.
Over the past three decades, tremendous progress has been made in epitaxial growth of semiconductors. Some landmark achievements include the demonstration of quantum well and superlattice structures and most recently, strained quantum well structures. As a result of these accomplishments, the performance of electronic and optoelectronic devices has been improved by orders of magnitude. Particularly in the last five years, optoelectronics research has been largely focused on short wavelength (yellow, green, blue and ultraviolet) light sources produced from high bandgap semiconductor epitaxial thin films. However, high-quality commercial devices, such as yellow, green, blue and UV diode lasers, can not be made without high-quality material platforms serving as substrates for the epitaxial layers. Thus, while dozens of new epitaxial compound materials have been recognized for various important applications, suitable substrate materials for growing these epilayers on other than conventionally used Si, GaAs, InP and sapphire substrates have yet to be perfected. Without the proper substrate, these epilayers will have too many defects to produce reliable devices. This is because a majority of these new compound semiconductor epilayers do not have the same lattice constant and crystal structure as any of the existing semiconductor substrates, so that high quality epitaxial layers are very difficult to achieve.
One seemingly straightforward solution to this problem is to synthesize new lattice-matched substrates for these new compound semiconductors. However, developing a new substrate is not only technologically difficult but is also extremely costly in most cases. Consequently, the very limited availability of substrate materials has severely limited the types of compound semiconductors people can use to make devices. This problem is expected to be worse in the future as the research of epitaxial compounds is becoming more diversified to produce more materials, but the research and development of semiconductor substrates is taking an opposite trend and is becoming more focused. The advance of semiconductor substrates in the past decades has mainly been in Si which is moving rapidly towards 12" wafers, and to a much less degree, in GaAs and InP where 3" and 4" substrates are available. However, the development for other substrates lags far behind these three materials in terms of wafer size and defect density.
A fundamental solution to this problem is to synthesize a "universal substrate" that can support defect-free epitaxial growth of any compound semiconductors of different lattice constants. It is even more desirable if this "universal substrate" can be formed from Si, GaAs or InP, the three best developed substrates. One innovative concept disclosed in the inventor's previous patent, U.S. Pat. No. 5,294,808, issued Mar. 15, 1994, is the use of a thin substrate having a thickness on the same order as the critical thickness (the thickness above which dislocations form when growing one lattice mismatched material on another). Because the substrate is extremely thin, it can be viewed as a compliant substrate. That is, instead of governing the lattice constant of the epitaxial layers, the thin substrate becomes flexible enough to adapt itself to the epilayers grown thereon.
Both theoretical and experimental results indicated that the concept of the compliant substrate is correct and could fundamentally solve the problem of finding proper substrate for new epitaxial materials. However, one drawback in the previous invention is that it does not present a proper way of forming a compliant substrate that is scaleable and robust enough for commercial applications. In particular, with a substrate membrane of only a few hundred angstroms thickness as proposed in the inventor's previous patent, it is difficult to sustain the mechanical and chemical processes required for epitaxial growth and device fabrication. Furthermore, even the ideal compliant substrate constructed in accordance with the teachings of the '808 patent may only work for thin films of a relatively small lattice mismatch (e.g., &lt;3%), but a lattice mismatch greater than 10% can often be found between semiconductor materials of significant interests. Therefore, to make compliant substrates more useful, not only is an effective implementation method needed, but a new type of compliant substrate is also needed which can accomodate large (e.g., 20%) lattice mismatch.